Various types and shapes of earth boring bits are used in various applications in today's earth drilling industry. The earth boring bits have bit bodies which include various features such as a core, blades, and pockets that extend into the bit body. Depending on the application, the drill bits may contain cutting elements such as polycrystalline diamond cutters (PDCs) and therefore be called PDC bits. Other bits have diamonds impregnated into the bit bodies for drilling through earthen formations. Such bits may also contain hot-pressed cutting elements called Grit hot-pressed inserts (GHIs). The cutting elements are received within the bit body pockets and are typically bonded to the bit body by brazing to the inner surfaces of the pockets. Bit bodies are typically made either from steel or from a tungsten carbide matrix. Bits made from the tungsten carbide matrix typically include a separately formed reinforcing member made of steel, and which is bonded to the matrix. The reinforcing member is positioned in the core section of the bit body and protrudes from the bit body.
The matrix bit body is typically formed of a single, relatively homogenous composition throughout the bit body. The single composition may constitute either a single matrix material such as tungsten carbide or a mixture of matrix materials such as different forms of tungsten carbide. The matrix material or mixture thereof, is commonly bonded into solid form by fusing a metallic binder material and the matrix material or mixture.
The drill bit formation process typically includes placing a matrix powder in a mold. The mold is commonly formed of graphite and may be machined into various suitable shapes. Displacements are typically added to the mold to define the pockets. The matrix powder may be a powder of a single matrix material such as tungsten carbide, or it may be a mixture of more than one matrix material such as different forms of tungsten carbide. The matrix powder may include further components such as metal additives. Metallic binder material is then typically placed over the matrix powder. The components within the mold are then heated in a furnace to the flow or infiltration temperature of the binder material at which the melted binder material infiltrates the tungsten carbide or other matrix material. This heating process is commonly referred to as sintering or liquid phase sintering. The infiltration process which occurs during sintering, bonds the grains of matrix material to each other and to the other components to form a solid bit body that is relatively homogenous throughout. The sintering process also causes the matrix material to bond to other structures that it contacts, such as a metallic blank which may be suspended within the mold to produce the aforementioned reinforcing member. After formation of the bit body, a protruding section of the metallic blank may be welded to a second component called an upper section. The upper section typically has a tapered portion that is threaded onto a drilling string.
The bit body typically includes blades which support the PDCs or GHIs which, in turn, perform the cutting operation. The blades may take on various shapes and may be reinforced with natural or synthetic diamonds formed on their respective surfaces, or they may be impregnated with diamond crystals throughout.
The drill bit body is typically formed to include cavities, commonly referred to as pockets, that extend into the bit body. The pockets which receive the cutting elements, are generally formed in the blade regions of the bit body.
The matrix material or materials determine the mechanical properties of the bit body. These mechanical properties include, but are not limited to, transverse rupture strength (TRS), toughness (resistance to impact-type fracture), hardness, wear resistance (including resistance to erosion from rapidly flowing drilling fluid and abrasion from rock formations), steel bond strength between the matrix material and steel reinforcing elements, such as a steel blank, and strength of the bond to the cutting elements, i.e., braze strength, between the finished body material and the PDC insert, GHI, or other cutting element. Abrasion resistance represents another such mechanical property.
The mechanical properties of the formed drill bit body may also be affected by the binder material used as well as the presence of diamond crystals impregnated within the bit body.
According to conventional drill bit manufacturing, a single matrix powder is selected in conjunction with the binder material, to provide desired mechanical properties to the bit body. The single matrix powder is packed throughout the mold to form a bit body having the same mechanical properties throughout. It would, however, be desirable to optimize the overall structure of the drill bit body by providing different mechanical properties to different portions of the drill bit body, in essence tailoring the bit body. For example, wear resistance is especially desirable at regions around the cutting elements and throughout the outer surface of the bit body, high strength and toughness are especially desirable at the bit blades and throughout the bulk of the bit body, superior braze strength is desirable in the pockets to which cutting inserts are brazed, and steel bond strength is desirable in the core region which is bonded to the steel blank. According to the conventional art, the choice of the single matrix powder represents a compromise, as it must be chosen to produce one of the properties that are desirable in one region, generally at the expense of another property or properties that may be desirable in another region.
It is therefore a shortcoming of the conventional art that a drill bit cannot be formed to include different desirable mechanical properties in different regions of the drill bit body. The present invention addresses these shortcomings.